This invention relates to an air gun firing sensor apparatus and system for monitoring the firing of an air gun used for the generation of seismic energy in a medium for determining the actual instant of firing of the air gun.
In seismic surveying, air guns are utilized to generate powerful seismic energy impulses in order to investigate subsurface geological conditions and formations in the earth. For this purpose, one or more such air guns are each charged with pressurized gas, usually compressed air, which is temporarily stored in them. At the desired "shot" instant, the air guns are fired, and the pressurized gas is abruptly released into the surrounding medium. In this manner, powerful seismic energy impulses are generated capable of penetrating deeply into subsurface material to be reflected and refracted by the various geologic strata and formations. The reflected and refracted impulses are sensed and recorded to provide information and data about the geological conditions and formations.
For many applications, in both marine and land seismic exploration, it has been found advantageous to generate a plurality of seismic energy impulses from an array of air guns in order to provide a composite seismic signal of predetermined amplitude and frequency content and in spacial or array arrangements. For marine seismic surveying an array of air guns having various firing chamber volumes may be utilized in order to produce a composite seismic signal having a predetermined frequency distribution, as air guns of different firing chamber volumes generate seismic energy impulses with different frequency spectra. In such applications, as well as those in which air guns are repetitively fired or fired in sequence, it is desired to control the timing of the solenoid control valves which are used to trigger the firing of the respective air guns.
For further background on air guns and solenoid valves, reference may be made to U.S. Pat. Nos. 3,249,177; 3,379,273; 3,779,335; 3,588,039; 3,653,460; 3,997,021 and 4,038,630.
In the past, attempts have been made to synchronize the timing of the solenoid-controlled valves which trigger the individual air guns. However, the time at which a solenoid plunger actually becomes actuated in response to an electrical signal will vary with each valve, and then there are further variables caused by the mechanical structure of each air gun, such as dimensional tolerance variations, wear of parts, slight differences in shuttle mass, different frictional drag of moving parts, and so on. When the individual air guns are not actually operating in accordance with the prescheduled precisely timed relationship, then the downgoing seismic waves are not in accordance with the predetermined survey program, and less than optimum results are obtained. For example, if the survey program calls for all of the air guns to fire in synchronism, and they are not synchronized, the downgoing seismic waves are misaligned. Misalignment may cause reduction in total amplitude of the downgoing seismic waves or cause the generation of seismic waves with undesirable frequency spectra, or both, resulting in data collection with reduced penetration and resolution.
A solution to the problem of inaccurate firing ("shot") instant as set forth in U.S. Pat. Nos. 4,034,827 and 4,047,591 assigned to Texas Instruments, Incorporated has been to sense movement of a gas-releasing shuttle within each air gun by magnetic, electromagnetic or electrostatic field effects to establish the precise instant of firing of each individual air gun. The firing signals applied to the respective air gun solenoid valves are then delayed or advanced relative to each other in subsequent firings to properly synchronize or sequence the firings of the respective air guns. In those patents, the air guns are modified in order that a field such as a magnetic field be set up. The air guns are further modified in order that variations in that field caused by movement of the shuttle can be detected.
In the final paragraph of each of the above patents, possible arrangements for producing a magnetic, electromagnetic or electrostatic field and sensing variations in that field were listed. A primary disadvantage of each of the embodiments suggested in these patents is that one or more complex modifications of the air gun itself is required. Such modification of an air gun to include means for generating a magnetic, electromagnetic or electrostatic field plus means to sense variations in that field are generally complicated procedures which cannot be accomplished by an operating crew, and even such modification of air guns yet to be manufactured is costly. Further, any repairs by an operating crew to a sensor which is built into the air gun itself is time-consuming and costly.
Another type of air gun seismic source transducer unit 10 is illustrated in FIG. 1 in the form of an electromagnetic sensor which is enclosed in a case and directly mounted on a solenoid valve 26 of an air gun in a position between the electric cable connector (not shown) and the solenoid valve. In effect, the relatively long sensor unit 10 is interposed between the solenoid valve 26 and the cable connector. This prior art sensor unit 10 includes a permanent magnet 11 mounted in a case of magnetically permeable material 12, an armature 14 having a T-shaped cross-section and a coil form 16 in which a coil 18 is wound. An annular spring 20 supports the armature mass 14 and spring biases the armature 14 away from the permanent magnet 11 in order to prevent the armature from being attracted into contact with a mechanical stop. This structure provides a rather short-stroke magnetic transducer for sensing the firing of the air gun. The spring mounting structure suffers the disadvantage of relatively short life due to mechanical fatigue resulting from the continuous stress of resisting magnetic attraction and from the repeated sudden shocks generated by the firings of the air gun which ultimately cracks and destroys the spring 20. Furthermore, such an annular spring may contain numerous oblique slots therein which cause stress concentrations and make the spring prone to fatigue failure. Also, once the sensor as shown in FIG. 1 is actuated, the spring suspension will tend to cause the relatively massive T-shaped armature to resonate or vibrate for a long period of time. In addition, the rather short initial stroke will generate a limited amplitude electrical signal which may not differ much from the subsequent signals generated by the continued vibrations of the mass 14 and spring 20 after the initial shock. It should also be pointed out that mounting a separate transducer unit 10 on top of the solenoid valve awkwardly elongates the overall assembly of solenoid valve-sensor unit-cable connector. Also, mounting the sensor unit 10 between the cable connector and the solenoid valve inherently requires two sets of plugin connections. One set of plug-in connections occurs between the cable connector (not shown) and the top end of the sensor unit 10. The second set of plug-in connections as indicated in dotted outline at 25 occurs between the sensor unit 10 and the solenoid valve 26. Such a duplication of plug-in connections increases the chance for faulty connections to occur and otherwise makes the sensor unit more vulnerable to damage by the powerful mechanical shocks generated when the solenoid valve is actuated and the air gun fired.
For more background on air guns in apparatus employed for seismic surveying on land, reference may be made to U.S. Pat. Nos. 3,310,128; 3,779,335; 3,800,907 and 4,108,271.